While optical communication systems are progressively increased in speed and capacity, optical modulators used in the optical communication systems are progressively increased in performance and density. In addition, in order to meet the demands of miniaturization in optical modulators, miniaturization of substrates constituting an optical modulator is in progress as well. However, since the degree of freedom of wirings design is small due to the limitation on the substrate size, the wirings on the substrate are required to be improved in order to achieve high performance, high density, and miniaturization of the optical modulator.
In regard to wirings for such an optical modulator, inventions have been proposed as follows.
For example, Patent Literatures 1 to 3 each disclose a structure in which a substrate having an optical waveguide formed in the substrate and a substrate amplifying, relaying, and terminating a high-frequency signal for modulation overlap each other in a separated manner.